[PATCH 4/5] perf dso: Guard against cache underflow on short reads in dso_cache__memcpy()

From: Arnaldo Carvalho de Melo <[email protected]>

dso_cache__memcpy() computes cache_offset = offset - cache->offset,
then cache_size = min(cache->size - cache_offset, size).  The RB tree
lookup in __dso_cache__find() matches using the full
DSO__DATA_CACHE_SIZE window, but cache->size reflects the actual pread
return value from dso_cache__populate().

A short pread (e.g. near end-of-file) makes cache->size smaller than
DSO__DATA_CACHE_SIZE.  If a subsequent access targets an offset past
cache->offset + cache->size but within the DSO__DATA_CACHE_SIZE
window, the cache entry is found but cache_offset exceeds cache->size.
Since both are u64, the subtraction cache->size - cache_offset wraps
to a large value, min() selects the caller's size, and memcpy reads
out of bounds.

Return 0 (cache miss) when cache_offset falls outside the valid cached
range, so the caller re-reads from the backing file.

Fixes: 366df72657e0 ("perf dso: Refactor dso_cache__read()")
Reported-by: sashiko-bot <[email protected]>
Cc: Adrian Hunter <[email protected]>
Assisted-by: Claude:claude-opus-4.6
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>
---
 tools/perf/util/dso.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/tools/perf/util/dso.c b/tools/perf/util/dso.c
index 845a384a4d56779b..2cf9f44a87903d7d 100644
--- a/tools/perf/util/dso.c
+++ b/tools/perf/util/dso.c
@@ -1005,7 +1005,17 @@ static ssize_t dso_cache__memcpy(struct dso_cache *cache, u64 offset, u8 *data,
 				 u64 size, bool out)
 {
 	u64 cache_offset = offset - cache->offset;
-	u64 cache_size   = min(cache->size - cache_offset, size);
+	u64 cache_size;
+
+	/*
+	 * The RB tree matches using DSO__DATA_CACHE_SIZE, but a short
+	 * pread may leave cache->size smaller.  Treat an offset past
+	 * the valid data as a cache miss so the caller re-reads.
+	 */
+	if (cache_offset >= cache->size)
+		return 0;
+
+	cache_size = min(cache->size - cache_offset, size);
 
 	if (out)
 		memcpy(data, cache->data + cache_offset, cache_size);
-- 
2.55.0